The invention relates to the mounting of sensors of physical quantities capable of working in harsh environments.
The mounting generally consists of the transfer of a micro-machined sensor to a base provided with electrical connection pins. The sensor is made, for example, out of several machined silicon wafers comprising mechanical elements (diaphragms, beams, seismic masses, etc), electronic elements (capacitor plates or strain gauges in particular), and metal contact pads used for electrical connection with the pins of the base when the sensor is fixed to the base.
Classically, the sensor is bonded or brazed by its rear face to the base, in a central part of this base that is surrounded by connection pins going through the base. The connection pads of the sensor, on the front face of this sensor, are connected by bonded wires between the connection pads and the tips of the connection pins that emerge from the surface of the base.
In this case, to ensure efficient operation in a harsh, wet or gaseous environment, it is necessary to cover the bonded wires, the connection pads and the ends of the pins with a protective insulating layer that prevents, firstly, the impairment of the sensor and, secondly, leakage currents between pins when the liquid or gas environment is not perfectly insulating. These leakage resistors indeed disturb the measurement of physical quantities which often relies on very small differential variations in resistance or on very weak electrical signals.
A polymerizable material, such as silicone resin, is then deposited on the conductive parts. Parylene may also be deposited. However, in applications such as pressure sensors, comprising a thin diaphragm in contact with the medium whose pressure is to be measured, it is necessary to avoid depositing this material on the diaphragm, because this would give rise to measurement errors and it is difficult to be aware of these errors and compensate for. Special precautions therefore have to be taken in depositing operations, and it may even be necessary to work by hand. Furthermore, this type of coating does not always withstand harsh environments.
The invention proposes to carry out an electrolytic deposition of metal on the conductive parts, followed by an operation for the oxidizing or nitriding of this metal so as to achieve the coating, with a layer of insulating oxide or nitride, of all the conductive parts that may subsequently come into contact with an ambient medium that is not perfectly insulating.
More specifically, the invention proposes a method for making a sensor of physical quantities consisting of the preparation of an active sensor part and a base, the active part comprising at least one wafer provided with conductive connection pads on one face and the base being provided with conductive pins, the electrical connection of the pads and the pins by conductive elements and then the plunging of the wafer and the pin ends into an electrolytic bath, the performance of an electrolytic deposition of at least one conductive metal on the pin ends, the pads and the conductive elements that connect them and the performance of an oxidizing or nitrizing operation on this metal to make an insulating coating on the connection pads, the connection pin ends and the conductive elements that connect them. The electrolytic deposit is made not only on the conductive parts but also on the insulating parts.
The term xe2x80x9celectrolytic depositxe2x80x9d is understood to mean a metal deposit (a single metal or an alloy or combination of metals deposited simultaneously or successively) on a conductive zone obtained by the migration of metal ions coming from a liquid solution. The migration may be prompted either by the passage of an electrical current (a classic electrolytic bath with current lead-in electrodes), or by chemical reaction (using what is called electroless deposition).
This method may be implemented either when the bonding wires are bonded between the pads and the pin ends or when the pin ends are each soldered directly to a respective pad.
The electrolytic deposit designed to be then oxidised or nitrized may be, for example, a tantalum deposit giving rise to a tantalum oxide or tantalum nitride coating that is especially resistant to chemical corrosion or to temperature and pressure.
The oxidizing will generally be carried out in a step subsequent to the step of electrolytic metal deposition, but it is sometimes possible to obtain the metal oxide directly during the electrolysis itself rather than to carry out a metal deposition and then an oxidizing operation in succession.